For example, high-temperature furnaces used in the steel industry, such as heating furnaces, blast furnaces, and heat-treating furnaces, include cylindrical members having a bend, a corner, or a curved surface, such as high-temperature pipes and walking-beam skid posts. For protecting and heat-insulating these members, inorganic fiber assemblies and inorganic fiber formed articles have been used. In particular, needled inorganic fiber assemblies (i.e., needled blankets) have been widely used taking advantage of their properties, such as low weight, excellent formability, excellent thermal shock resistance, excellent wind erosion resistance, and low thermal conductivity. When needled blankets are applied to a target that is to be protected, they are compressed into formed articles, which are formed into a shape like a ring or a cut ring and subsequently fit on the target so as to be stacked on top of one another.
Heat-insulating protective members may be corroded by scale and alkaline gas generated in furnaces. In particular, in heating furnaces used in the steel industry, heat-insulating protective members may be physically damaged from iron oxide present in the furnaces. Furthermore, the inorganic fibers may form low-melting compounds, which act as sources of erosion and embrittlement. As a result, heat-insulating protective members may be degraded at an early stage.
In order to address the above issues, there have been reported various inorganic fiber formed articles that include an inorganic fiber assembly to which an inorganic sol, a binder, and the like are added.
For example, PTL 1 describes an inorganic fiber block having high corrosion resistance which is produced by applying an alumina sol or a mixed sol including an alumina sol and a silica sol onto a surface oriented in the direction in which blankets are stacked such that the amount of the alumina sol or the mixed sol deposited is 55 to 300 g/m2 in terms of solid component and drying the resulting coating layer.
PTL 2 describes an inorganic fiber block provided with a coating layer having a thickness of 2 mm which is formed by spraying an anti-FeO coating material onto the surface of the inorganic fiber block placed in a furnace. It is also described that the coating layer includes particles of CA6 (CaO.6Al2O3).
PTL 3 describes a lightweight inorganic fiber formed article having a bulk density of 0.08 to 0.20 g/cm3 which is produced by impregnating a needled blanket composed of inorganic fibers with an inorganic sol and drying the impregnated needled blanket.
PTL 4 describes a refractory covering material capable of being applied without degrading flexibility which is produced by depositing cement on a felt-like mat composed of refractory fibers and moistening the mat by spraying water to the mat or immersing the mat in water. PTL 5 describes a technique in which a wet fiber felt material prepared by impregnating inorganic fibers with colloidal silica serving as an inorganic binder is arranged on the surface of a steel beam. PTL 6 describes a wet refractory heat-insulating material produced by forming a water slurry including inorganic fibers, a binder, and an inorganic powder into a shape.
PTL 7 describes a method in which a needled blanket composed of inorganic fibers is impregnated with an inorganic sol, the impregnated needled blanket is dried to form a cylindrical inorganic fiber formed article, which is divided into pieces in the direction parallel to the shaft center of the cylinder, and the pieces are arranged on a cylindrical member.
PTL 1: JP H11-211357 A
PTL 2: JP 2011-32119 A
PTL 3: JP 2011-208344 A
PTL 4: JP S60-112947 A
PTL 5: JP S63-194051 A
PTL 6: JP S62-288178 A
PTL 7: JP 2014-5173 A
In the inorganic fiber blocks described in PTLs 1 and 2, the coating layer formed by the application or spraying of the coating material are difficult to permeate into the inside of the fibers and likely to solidify on the surfaces of the inorganic fibers. Thus, the coating layer is likely to detach from the inorganic fiber block formed article. Accordingly, the coating layer is likely to detach from the inorganic fiber block formed article due to thermal shock, mechanical shock, or the like and, as a result, the inorganic fibers present inside the inorganic fiber block may be exposed. In PTL 2, after the inorganic fiber formed article has been applied to walls of a furnace, a coating material is applied to the walls of the furnace with a spray gun. Thus, the process for applying the inorganic fiber formed article to the furnace is complex. In addition, after the spraying of the coating material and the following drying-firing step have been terminated, the surface of the inorganic fiber block becomes hard, and the thermal shock resistance becomes degraded.
The inorganic fiber formed article described in PTL 3 is produced by impregnating the entirety of the needled blanket with the inorganic sol and drying the impregnated needled blanket. Therefore, the inorganic fiber formed articles are inflexible and not capable of being tightly arranged on a bend, a corner, a curved surface, or the like without any gap therebetween.
The refractory covering materials described in PTLs 4 and 5 are flexible since they are in a wet state. However, in PTL 4, cement is deposited on the mat, which is subsequently moisturized. This requires a complex process. In PTL 5, where colloidal silica is used as an inorganic binder with which inorganic fibers are impregnated, it is not possible to achieve sufficient scale resistance.
Moreover, moisture content is not discussed in PTLs 4 and 5. Thus, in PTLs 4 and 5, the refractory covering materials are heavy, poor in workability, and difficult to be fixed to a member that is to be protected. The lack of control of wetness may result in a phenomenon referred to as “migration” in which the inorganic binder is solidified on the surface when drying is performed. Furthermore, the amount of deposited binder may become excessively large. This increases the local density of the surface portion and thermal shrinkage ratio. In addition, the thermal shock resistance may be degraded. This results in cracking and detachment of the surface. In the application methods described in the patent literatures above, moreover, gaps may be created therebetween when being heated and shrunken. This significantly reduces the heat-insulating property and degrades the scale resistance (resistance to FeO).
The refractory heat-insulating material described in PTL 6 is flexible since it maintains a slurry form. However, the refractory heat-insulating material has a low mechanical strength and is likely to be torn when being applied to a target that is to be protected.
The inorganic fiber formed article described in PTL 7 is capable of being tightly arranged on a member having a cylindrical shape or the like without any gap therebetween. However, since the inorganic fiber formed article has low flexibility, it becomes impossible to apply the inorganic fiber formed article to the member that is to be protected when the cylindrical shape was changed by deformation. Since members that are to be protected in heating furnaces are particularly likely not to have a consistent shape but deform due to erosion by scale or deformation by heat, it is not possible to apply the inflexible inorganic fiber formed article to such members.